1. Field of the Invention
The present invention relates to semiconductor laser devices used, for example, for optical communication apparatuses and electrophotographic printers, and more particularly, relates to a semiconductor laser device having improved multilayer mirrors used as a resonator.
2. Description of the Related Art
A surface emitting laser is one type of semiconductor laser developed to obtain light in a direction perpendicular to a semiconductor substrate. A great number of surface emitting lasers may be integrally arranged to form a two-dimensional array. Hence, when a surface emitting laser array is used as an exposure light source of an electrophotographic printer, by parallel processing of a printing step using a multibeam system, an increase in printing speed can be realized.
A surface emitting laser currently practically used is an element which emits laser light in an infrared region. However, when the oscillation wavelength is shortened from the infrared region to a blue or an ultraviolet region, the beam diameter can be decreased and a higher resolution can be obtained. Hence, surface emitting lasers which can be practically used from a blue to an ultraviolet region have been requested. Effects obtained by combination between a higher resolution caused by shortening a wavelength and parallel processing caused by using a multibeam system are significant, and besides the application to printers, expansion of applications for various fields can also be expected.
The surface emitting laser has a cavity provided perpendicular to an in-plane direction of a substrate, and in order to realize a surface emitting laser continuously operable at room temperature, a mirror capable of ensuring a reflectance of 99% or more must be used.
As the mirror as described above, a multilayer mirror has been used in which two types of materials having different refractive indices and having, for example, an optical thickness corresponding to ¼ wavelength are alternately laminated to each other at least two times.
In a near-infrared surface emitting laser using a GaAs-based semiconductor, which has been practically used, a semiconductor multilayer mirror is used in which GaAs and AlAs layers having extremely high crystallinity are provided in combination as constituent layers. In addition, a semiconductor multilayer mirror is also used in which AlGaAs containing a small amount of Al and AlGaAs containing a large amount of Al are provided in combination as constituent layers.
In addition, a semiconductor multilayer mirror used in a GaN-based semiconductor laser capable of emitting light of a shorter wavelength than that of a GaAs-based semiconductor laser has been studied and disclosed, for example, in Japanese Patent Laid-Open No. 7-297476, 4th International Conference on Physics of Light-Matter Coupling in Nanostructures, W4-1, 2004, St. Petersburg, Russia, and Appl. Phys. 86, 031107 (2005).
In order to obtain a high reflectance by a semiconductor multilayer mirror, it is necessary to increase the difference in refractive index between a material forming a high refractive-index layer and a material forming a low refractive-index layer and, in addition, to increase the number of lamination pairs each containing the high refractive-index layer and the low refractive-index layer. When the semiconductor multilayer mirror is formed by epitaxial growth, in order to suppress the lattice strain, the difference in lattice constant between the materials is preferably small. The reason for this is that when the cumulative lattice strain amount is increased, concomitant with an increase in the number of pairs, cracking may occur.
As for GaAs and AlAs forming a multilayer mirror used in a GaAs-based semiconductor laser, as shown in FIG. 14, the difference in lattice constant is very small (approximately 0.14%), and the difference in refractive index is sufficiently large (approximately 15%). Hence, in the GaAs-based semiconductor laser, a semiconductor multilayer mirror having a high reflectance can be obtained.
On the other hand, as for a semiconductor multilayer mirror used in a GaN-based semiconductor laser, the reflectance is difficult to be improved.
FIG. 15 shows the relationship between the lattice constant and the refractive index of material compositions used for a semiconductor multilayer mirror of a GaN-based semiconductor laser. In order to increase the difference in refractive index, when GaN is used for a high refractive-index layer and AlGaN containing an increased amount of Al is used for a low refractive-index layer (in a direction shown by the arrow in the figure), the degree of lattice mismatch is increased. For example, in order to increase the difference in refractive index, when several tens of pairs of GaN and AlN are grown on a GaN substrate, a large tensile strain is generated in an AlN layer in the multilayer film, and as a result, cracking occurs in the miller. As described above, when AlN or AlGaN containing an increased amount of Al is used, since the number of pairs to be laminated cannot be increased, it becomes difficult to achieve a reflectance of 99% or more.
As one method to solve the above problem, it has been investigated that instead of AlN and/or AlGaN, AlInN that can be lattice-matched with GaN is used for a constituent layer of a semiconductor multilayer mirror.
In Japanese Patent Laid-Open No. 7-297476, a semiconductor multilayer mirror formed of pairs of GaN/AlInN has been disclosed. In addition, in 4th International Conference on Physics of Light-Matter Coupling in Nanostructures, W4-1, 2004, St. Petersburg, Russia, and Appl. Phys. 86, 031107 (2005), it has been disclosed that when 40 pairs of GaN (high refractive-index layer) and Al0.83In0.17N (low refractive-index layer) having a lattice match with GaN are formed, a high reflectance of 99.4% is realized.
However, by research carried out by the inventor of the present invention, it was finally understood that when GaN is used for a high refractive-index layer and Al0.83In0.17N is used for a low refractive-index layer, since light absorption in an element is high, further improvement must be performed.